RFC: Erroring out if the platform you're building the Framework is not supported

[Albert Astals Cid]

Supported == listed in metainfo.yaml

https://invent.kde.org/frameworks/extra-cmake-modules/-/merge_requests/245

What do you think?

I know parsing yaml like that isn't great but it seems to work for something as simple as what I want there.

I guess it's probably a bit annoying for places like OpenBSD where having Linux and FreeBSD supported probably means it would also work, but IMHO they can either just use -DIGNORE_PLATFORM_CHECK=true or work with us so we get an OpenBSD CI and then we add it to the metainfo.yaml list.

Cheers,
  Albert